1. Field of the Invention
The present invention relates to an outboard motor having a mechanism arranged to transmit engine power to a propeller through a shaft, and especially relates to an outboard motor equipped with a transmission arranged to change the rotational speed of a shaft.
2. Description of the Related Art
Conventionally, in an outboard motor, engine power is transmitted to a crankshaft, a drive shaft, and a propeller shaft, and then is transmitted to a propeller from the propeller shaft. Conventionally, the engine power is transmitted as is to the propeller through each of the aforementioned shafts. Thus, when it is desired to change the speed of a watercraft, the engine power is adjusted to change the rotational speed of each shaft, so that the rotational speed of the propeller is changed subsequently to cause a change in the speed of the watercraft.
However, recent outboard motors have been provided with a transmission on a drive shaft and adapted to change the speed of a watercraft with the transmission rather than relying exclusively on the engine power (see WO 2007/007707, for example).
It often happens that the upper casing in which the drive shaft is inserted does not have enough width to allow for the insertion of the transmission. Thus, when the transmission is mounted on the drive shaft, a wide upper casing is utilized so that the drive shaft and the transmission can be inserted therein. In addition, with modifications of the upper casing, other members such as a mounting plate attached above the upper casing and a lower casing attached below the upper casing need to be modified in accordance with the configuration of the upper casing. This causes a problem of increased cost.
Furthermore, when the lateral width is widened from a lower section of the upper casing through the lower casing for the insertion of the transmission in the upper casing, there arises a problem of increased resistance to water.